The mainstream of semiconductor industry progress is primarily driven by the ability of shrinking the size of basic structures of an integrated circuit (IC), for example, the metal oxide semiconductor field effect transistor (MOSFET). New materials of peculiar electrical and physical characteristics, in addition to the traditional materials of microelectronics and development of new concepts of operation, are considered essential in the latest version of the International Technology Roadmap for Semiconductors (ITRS)-2005 Edition, available from www.itrs.org. Among the identified materials, CNT devices are emerging as fundamental building blocks for a potentially new, cost-effective, nano-electronics science.
Since their discovery by S. Iijima, see for example, “Helical microtubes of graphitic carbon”, Nature vol. 354, pp. 56-58, (1991), CNTs have been a popular research topic for their unique chemical, physical, and electrical properties. A few properties of great impact on micro- and nano-electronics applications are: (1) metallic and semiconducting electrical behavior with size in the nanometer scale length; (2) outstanding charge transport properties due to intrinsic mono-dimensionality that drastically reduces scattering and consequent power dissipation; (3) chemical passivation of their surface is generally not required, thus allowing use of a high-κ dielectric; and (4) chemical and thermal stability and resistance to electro-migration at a current density in the order of 109 A/cm2.
Detailed reviews on these topics can be found in M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of fullerenes and Carbon Nanotubes, Academic Press (1996), P. Avouris, J. Appenzeller, R. Martel and S. J. Wind “Carbon Nanotube Electronics” Proced. IEEE Vol. 91, N 11, Nov. 2003, in “Carbon Nanoture Electronics and Optoelectronics” published in MRS Bulletin June 2004 page 403 by P. Avouris, or in “Properties and applications of high-mobility semiconducting nanotubes” published by T. Dürkop, B. M. Kim and M. S. Fuhrer in J. Phys.: Condens. Matter 16 (2004) R553-R580 and “Physics of carbon nanotube electronic devices” by M. P. Anantram and F. L'eonard in Rep. Prog. Phys. 69 (2006) 507-561.
A major hurdle to be overcome for CNTs to find prominent uses in ICs is to develop fabrication techniques that may be compatible with present ICs fabrication processes. The IC fabrication processes are based on few elementary steps: films are deposited onto a wafer and pattern-etched away through photolithographic definition steps. The technique of fabricating nanotubes and/or nanowires that has the potential of enabling their integration in solid state devices appears to be based on Catalyzed Chemical Vapor Deposition (CCVD). The formation process starts from patterned catalyzed areas of a substrate, over which nano and/or sub-nano particles of a catalyst (promoter), usually nano-particles of transition metals such as of iron (Fe), carbon monoxide (Co), nickel (Ni), molybdenum (Mo) and alloys of the metals, are finely dispersed in order to act as initiation sites of CNTs nucleation. Usually a “flash” deposition of the metal catalyst over the surface of the substrate is effective in establishing thereon a uniform dense population of closely spaced (discrete) nanoparticles constituting the nucleation sites of CNTs over the catalyzed area.
Different hydrocarbons such as benzene (C6H6), pentane (C5H12), acetylene (C2H2), methane (CH4) and even carbon monoxide (CO), may be catalytically decomposed at temperatures between 500 and 1200° C. in the presence of a carrier gas like H2, Ar, and NH3. The method allows both single-wall nanotube (SWNT) and multi-walled nanotube (MWNT) growth with the process at different ranges of temperatures. See for example, “Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon based substrates”, R. Angelucci, R. Rizzoli, M. F. Bevilacqua, V. Vinciguerra, submitted to Journal of Physical E: Low-dimensional Systems and Nanostructures (2006); “Carbon Nanotubes Grown by Catalytic CVD on Silicon Based Substrates for Electronics Applications”, R. Rizzoli, R. Angelucci, S. Guerri, A. Parisini, G. P. Veronese, V. Vinciguerra, M. F. Bevilacqua, submitted to Advanced Materials Research (2006); “Patterned growth of carbon nanotubes synthesized by Fe-catalyzed chemical vapor deposition”, V. Vinciguerra, M. F. Bevilacqua, R. Angelucci, R. Rizzoli, Chemistry Today, October 2004; and “Carbon Nanotubes: Synthesis and Applications”, R. Angelucci, R. Rizzoli, F. Corticelli, A. Parisini, V. Vinciguerra, M. F. Bevilacqua, L. Malferrari and M. Cuffiani, IAEA Technical Report, April 2004.
The CCVD process conditions can be those of thermal activation (CVD), plasma enhancement (PECVD) or microwave enhancement (MWCVD). See for example, “Carbon nanotube growth by a review” by M. Meyyappan, Lance Delzeit, Alan Cassell and David Hash, published in Plasma Sources Sci. Technol. 12 (2003) 205-216; and “Growth of carbon nanotubes by thermal and plasma chemical vapor deposition processes and applications in microscopy” by Lance Delzeit, Cattien V. Nguyen, Ramsey M. Stevens, Jie Han and M. Meyyappan in Nanotechnology 13 (2002) 280-284. During the growth, an external electric field is applied to direct the growth of the tubes along field lines from negative to positive polarity. See H. B. Peng et al., APL (2003) Vol. 83 page 4238.
It is also known that by using a substrate/buffer-layer/catalyst-layer stack, a certain control of the growth of CNTs can be implemented on the basis of pre-conditions established by stack composition. The metal catalyst can be deposited by physical procedures (sputtering, e-gun, or resistive evaporation) or by chemical solutions (starting from precursors of the metal catalyst), and eventually patterned in well-defined areas by an appropriate mask. Depending on the method and conditions of the catalyst deposition, the size of the CNTs may be substantially controlled. Whether the catalyst is sputtered, deposited from vapor phase or by thermal reduction of a decomposable salt solution applied on the surface to be catalyzed, the more the amount of the catalyst, the larger the size of deposited metal particles or of metal particle clusters, and generally larger will be the diameters of the grown CNTs. In any case the amount of catalyst metal is kept(the “thickness” of the deposited catalyst layer) well below a certain critical thickness that generally may be between 10 and 1000 Å. Size and density of the catalyst particles play an important role, together with the carbon deposition conditions, in the formation of either single or multi-wall shell nanotubes. See, for example, “Diameter-controlled synthesis of Carbon Nanotubes”, J. Phys. Chem. B 106 (2002) 2429-2433 by Chin Li Cheung, Andrea Kurtz, Hongkun Park, Charles M. Lieber. If the catalyst is deposited by chemical deposition, the concentration of precursor salts of the catalytic metal in the solution usually controls the average sizes of the deposited metal particles that determine the sizes of the grown nanosized filamentary carbon structures (CNTs).
PCT Patent Application Publication No. WO2005102922 to Philips, and the article “A self-assembled synthesis of carbon nanotubes for interconnects”, Nanotechnology, 17, (2006) pages 1062-1066, by Zexiang Chen, Guichuan Caol, Zulun Lin, Irmgard Koehler and Peter K Bachmann, disclose how to grow highly oriented, freestanding and structured CNTs by Plasma-Enhanced CVD. According to this method, an array of multi-layered structures, each multi-layer including a substrate, a bottom electrode, a buffer layer, a catalyst layer, a second buffer layer and a top electrode, is defined by standard deposition techniques. A MWPECVD is then used to activate nucleation and growth of the CNTs. The technique permits lifting of the top electrode during the growth phase. The technique suffers in that the heavy top electrode may distort the growing CNTs and is scarcely equipped to determine, a priori, the exact lengths of the grown CNTs when fabricating multiple structures (CNT devices), a requisite for mass-production applications. Moreover the use of metal electrodes for biasing the stack imposes a maximum temperature limit to the CNT's growth process.
In CVD growth processes, application of an electric field assists the nucleation and orients the growth along the field lines, typically from lower to higher potential. See, for example, “Electric-field-directed growth of aligned single-walled carbon nanotubes”, Y. Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E. Yenilmez, J. Kong and H. Dai, Appl. Phys. Lett. 79, 3155-3157 (2001), “Vectorial growth of metallic and semiconducting Single-Wall Carbon Nanotubes”, E. Joselevich and C. Lieber, NanoLetters 2, 1137-1141 (2002), “Electric-field-aligned growth of single-walled carbon nanotubes on surfaces”, A. Ural, Y. Li and H. Dai, Appl. Phys. Lett. 81, 3464-3466 (2002), and “Electric-field-directed growth of carbon nanotubes in two dimensions”, A. Nojeh, A. Ural, R. F. Pease and H. Dai, J. Vac. Sci. Technol. B 22, 3421-3425 (2004).